feat(gemma): initial implementation of Gemma model and configs

- Add core Gemma model (architecture, attention, GeGLU, RoPE, RMSNorm, etc)
- Add configs for training and generation: gemma_train.json, gemma_generate.json
- Add Gemma notebook for exploratory analysis and demonstration
- Add __init__.py for Gemma submodule
- Update run_llm_experiment.py to support Gemma experiment configs

test(gemma): add comprehensive unit tests for Gemma

- Test forward pass (with/without cache)
- Test autoregressive generation (greedy, top-k, top-p)
- Test shape correctness and max sequence length errors
- Test multi-layer stack and token embeddings

docs: add documentation notebook for Gemma usage and analysis

Closes: #issue (if applicable)

llm/tests/models/test_gemma.py

@@ -0,0 +1,56 @@
+# llm/tests/models/test_gemma.py
+
+import torch
+import pytest
+from llm.models.gemma.gemma import Gemma
+
+@pytest.fixture
+def config():
+    return {
+        "vocab_size": 100,
+        "embed_dim": 32,
+        "num_q_heads": 4,
+        "num_layers": 2,
+        "max_position_embeddings": 16,
+        "dropout": 0.0,
+    }
+
+@pytest.fixture
+def model(config):
+    return Gemma(config)
+
+def test_forward_basic(model):
+    x = torch.randint(0, 100, (2, 8))
+    logits, cache = model(x)
+    assert logits.shape == (2, 8, 100)
+    assert isinstance(cache, list)
+    assert len(cache) == model._decoders.__len__()
+
+def test_forward_with_cache(model):
+    x = torch.randint(0, 100, (2, 4))
+    logits, cache = model(x, use_cache=True)
+    # Второй проход с cache и одним новым токеном
+    x2 = torch.randint(0, 100, (2, 1))
+    logits2, cache2 = model(x2, use_cache=True, cache=cache)
+    assert logits2.shape == (2, 1, 100)
+    assert isinstance(cache2, list)
+
+def test_generate_and_shape(model):
+    x = torch.randint(0, 100, (1, 5))
+    result = model.generate(x, max_new_tokens=3, do_sample=False)
+    assert result.shape == (1, 8)
+
+def test_forward_sequence_too_long(model, config):
+    x = torch.randint(0, 100, (1, config["max_position_embeddings"] + 1))
+    with pytest.raises(ValueError):
+        model(x)
+
+def test_generate_with_sampling_topk(model):
+    x = torch.randint(0, 100, (1, 3))
+    out = model.generate(x, max_new_tokens=2, do_sample=True, top_k=5)
+    assert out.shape == (1, 5)
+
+def test_generate_with_sampling_topp(model):
+    x = torch.randint(0, 100, (1, 3))
+    out = model.generate(x, max_new_tokens=2, do_sample=True, top_p=0.8)
+    assert out.shape == (1, 5)